for all those people who are having a problem like this one.
It seems the culprit is not a hack but rather a virus which snoops on your FTP password, logs-in on your account and try to append this malicious script on your .php files inside your webfolders
see this link: blackhatworld.com/blackhat-seo/black-hat-seo/158192-new-hack-wordpess-blogs-parese-error-wp-includes-default-widgets-php-line-1035-a.html